Lithium-ion (Li-ion) batteries, expected to have a global market value of $47 billion by 2023, are used in numerous applications because they offer relatively high energy density (storage capacity), high operating voltage, long shelf life, and little “memory effect” (a reduction in a rechargeable battery's maximum capacity due to incomplete discharges in previous uses). However, factors such as safety, charge/discharge cycling, and operating life continue to limit the effectiveness of Li-ion batteries in heavy-duty applications, such as powering electric vehicles.

Scientists from the University of Virginia (UVA) are employing neutron imaging techniques at Oak Ridge National Laboratory (ORNL) to probe Li-ion batteries and obtain insights into the electrochemical characteristics of the batteries’ materials and structures. Their research, published in the Journal of Power Sources, focused on tracking the lithiation/delithiation (charge/discharge) processes in Li-ion battery electrodes using thin and thick sintered samples of two electroactive materials, lithium titanate (LTO) and lithium cobalt oxide (LCO).

“When electrodes are relatively thick, transport of lithium ions through the porous material and separator architecture can limit charge and discharge rates,” said Gary Koenig, a professor in UVA’s Department of Chemical Engineering. “To develop methods of improving Li-ion transport through an electrode’s porous void regions filled with electrolyte, we need to first be able to track the transport and distribution of the ions within a cell during the charge and discharge processes.”

According to Koenig, other techniques such as high-resolution x-ray diffraction can provide detailed structural data during electrochemical processes, but that method typically averages relatively large volumes of the material. Similarly, x-ray phase imaging can visualize salt concentrations in battery electrolytes, but the technique requires a special spectrochemical cell and can access composition information only between the electrode regions.

To obtain detailed information across a wider area, the researchers conducted their studies using neutrons at the cold neutron imaging beamline at ORNL’s High Flux Isotope Reactor.

“Lithium has a large absorption coefficient for neutrons, which means that neutrons passing through a material are highly sensitive to its lithium concentrations,” “Neutrons are highly sensitive to lithium, which changes their paths and energies as they pass through a sample cell before reaching detectors that measure the changes,” said Ziyang Nie, lead author and graduate student in Koenig’s group. “We demonstrated we could use neutron radiographs to track in situ lithiation in thin and thick metal oxide cathodes inside battery cells. Because neutrons are highly penetrating, we did not have to build custom cells for the analysis and were able to track the lithium across the entire active region containing both electrodes and electrolyte.”

Comparing the lithiation process in thin and thick electrodes is essential to aid understanding of the effects of heterogeneity—local variations in mechanical, structural, transport, and kinetic properties—on battery life and performance. Local heterogeneity can also result in nonuniform battery current, temperature, state of charge, and aging. Typically, as the thickness of an electrode increases, so do the detrimental effects of heterogeneity on battery performance. Yet if thicker anodes and cathodes could be used in batteries without impacting other factors, it would help increase energy storage capacities.

For the initial experiments, the thin electrode samples had thicknesses of 0.738 mm for LTO and 0.463 mm for LCO; the thick LTO and LCO samples were 0.886 mm and 0.640 mm, respectively.

“Our immediate goal is to develop a model to help us understand how modifying the structure of an electrode, such as changing how the material is oriented or distributed, could improve ion transport properties,” said Koenig. “By imaging through each sample at different points in time, we were able to create 2D maps of lithium distribution. In the future, we plan to rotate our samples within the neutron beam to provide 3D information that will reveal in more detail how heterogeneity impacts ion transport.”

The High Flux Isotope Reactor is a DOE Office of Science User Facility. UT-Battelle LLC manages ORNL for DOE’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov/.—by Paul Boisvert